Fabricio's been lookin' pretty good upon his return to the UFC. I think Cain's faster on the feet, and would give Werdum problems there, despite Werdum's improved muay thai.
Unless you've got Dos Santos KO power, you probably won't finish Werdum... I can't think of someone whose guard I'd be less inclined to drop into to try and finish someone.
I'd like to see Werdum v. Barnett.